A standard motor-vehicle door latch has a housing attached to a door edge and holding a pivotal fork that can move between a latched position engaged around a bolt extending from a doorpost and an unlatched position allowing the latch to move relative to the bolt. When latched, the door is held closed, when unlatched it can be opened. As a rule the fork is retained in the latched position by a pawl which is pivotal to release the fork and which itself is displaced by a release lever. An actuating lever moved by an inside and/or outside door handle is coupled through respective inside and outside operating levers with the release lever to operate same and unlatch the door. Another locking lever is provided that can, in a locked position, set the mechanism of the latch so no movement is transmitted from the actuating lever to the release lever and, in an unlocked position, set the mechanism so the actuating lever operates the release lever. See EP 1,070,816 of Bland and EP 1,039,079 of Hochart.
It is standard to form the door latch with a base plate acting as a main housing element and in which a plurality of pins are permanently set, e.g. by riveting. The various pivotal elements of the latch mechanism—the fork, pawl, release lever, operating levers, actuating lever, and locking lever—are carried on these pins and interact with each other and with external elements through slots in the housing plate. It is also standard for the outer ends of these pivot pins to be seated in another housing plate so as to rigidify the assembly and enclose the mechanism.
In normal use considerable radial forces are exerted on the pins traversing the two plates. For instance, when the door is closed its entire mass is effective as a torque on the locking fork which is converted by the latching pawl into a force radial of its axis, often in a direction tending to push the fork's pivot away from the pawl's pivot. Since the various levers and elements of the latch mechanism are often made as simple stampings or castings and do not conform dimensionally to high tolerances, these forces can vary considerably. In time they can deform the housing assembly comprised of the plates and pivot pins, changing the spacing and/or the relative angular orientation of the pivot pins. The latch can misfunction either by opening when it should not or by not opening when it should.
When the car door held by the latch is subjected to violent movement, as for instance when traveling on a very bumpy road or in an accident, the forces pushing the pivot pins apart can be even more extreme and lead to immediate unwanted failure of the latch. The classic problem is a door that refuses to open after the vehicle has been in an accident.